Joining the fray:
We are always on the lookout for a place to stay for little or no money when we are on the move. Walmart is most suitable and several have areas set aside for RVs usually near the store on the side by the car service area. Little traffic there overnight. We are Elks so that is an option and Bass Pro and Cabellas have welcomed us and our shopping dollars more than once. I would think a Police Station would be dire emergency only as an option, although very safe.
There is a subscription website
http://www.overnightrvparking.com/index.php that has the most uptodate listing of available WalMarts and other free parking.
I try to park near an edge so I can open the bedroom slide for comfort.